2. How Folds Form Flashcards
What is the difference between buckling, bending and passive folding?
Buckling: folding that occurs when layers are loaded parallel to the layering
Bending: folding that occurs when layers are loaded at a high angle to layering
Passive Folding: folding of marker bands that cross or are contained within a deforming unit
How does the wavelength of a single-layer buckle fold change with layer thickness and with the layer: matrix viscosity contrast?
Fold wavelength increases as layer thickness increases
large viscosity contrasts lead to more rapid fold amplification and less layer thickening
Draw the shape of a folded pinch-and-swell layer (e.g., top right photo on Slide 7). Have a go at drawing the axial trace on this sketch.
What is the distinction between harmonic, polyharmonic, disharmonic and ptygmatic folding?
Harmonic: competent layers are a little further apart than effective single layer
Polyharmonic: When the competent layers vary markedly in thickness and/or in viscosity contrast with the matrix - folds with more than one wavelength develop
Disharmonic: competent layers are further apart than each other’s zone of influence ( about one wavelength) - each layer folds independently
Ptygmatic: relatively incompetent layers are much thicker than the competent layers - fold axial surfaces vary greatly in orientation
What is an accommodation structure and why does it arise?
structures that form to allow limbs that are too long or short to adjust when the controlling layer starts to buckle
Sketch a sinistral contractional kink band. Draw a sinistral extensional kink band. Draw the dextral variants of these. Draw a conjugate pair of contractional kink bands. Draw a conjugate pair of extensional kink bands.
What are the four stages in the development of a fold?
- Pre-buckle shortening (layer thickening)
- Fold initiation (nucleation)
- Fold amplification → locking up
- Post-buckle flattening (kinematic growth)
What is a flexural slip fold? What happens (on a stereogram) to a pre-existing lineation when it is deformed by flexural slip folding?
Flexural slip is like compressing a ream of paper parallel to the sheets - individual sheets slide over each other
- maximum slip is at the infelction points and decreases towards the hinge where it is zero
- there is no distortion within the plane of the layering
on a sterogram, folding spreads the lineation along a small circle about the fold axis
What is a tangential longitudinal strain fold? What does the neutral surface separate?
Tangential Longitudinal Strain (TLS) folding is like compressing an eraser parallel to its length - the outer arc of the layer experiences extension and the inner arc experiences compression; between them is the neutral surface where the deformation cancels out
the maximum deformation is at the hinge → zero at the inflection point
Secondary features: tension gashes in the outer arc, intensified cleavage or conjugate thrusts in the inner arc
What is a shear fold? A pre-existing lineation is spread along a great circle (on a stereogram) when it is deformed by shear folding. What is the significance of the intersection of that great circle with the great circle describing the shear plane?
Shear folding is like shearing a pack of cards: the shear planes are parallel to the axial surface of the fold
The intersection of the great circle with the shear plane (axial plane) us the tectonic a-direction - the orientation of the fault slip vector
What is a pericline? What is a sheath fold? What is a fold interference pattern?
Folds grow and die out along the fold axis or the fold hinge may bifurcate -a common spatial arrangement is that of elongate domes (periclines) arranged in an en echelon array
Sheath folds: folds with hinge lines that curve through more than 90° within the axial surface - a feature of shear zones (class 4)
There are four basic fold interference patterns based on the orientation of the fold axis and the axial planes of the two phases of folding relative to each other
